Effect of acidifying amendments on P availability in calcareous soils

Phosphorus (P) reactions in calcareous soils limit the concentration of P in the soil solution for plant uptake. Calcareous soils with high calcium carbonate content (CaCO 3 ) and high pH have low P fertiliser efficiency, leading to crop deficiency and limited crop productivity. The aim of this work was to test if soil acidifying amendments could reduce soil pH and improve the solubility of fertiliser P to improve crop P nutrition and biomass. Three calcareous soils with varying CaCO 3 content (14–29% w/w) were used to test acidifying amendments both with and without mono-ammonium phosphate (MAP). Soil was amended with oxalic acid, sulfuric acid, glucose, ammonium sulfate and elemental sulfur (S 0 ). Initial titrations demonstrated the ability of oxalic and sulfuric acids to reduce pH and improve P solubility in all three soils. Incubation of the acidifying amendments in the soil with the lowest carbonate content over 14 days (52 days for S 0 ) showed increased P solubility and diffusion from MAP granules in soil amended with glucose, sulfuric acid and oxalic acid. There was, however, no improvement in P fertiliser uptake in wheat crops grown in these highly calcareous soils.